Charging process for electrostatic screening of color tubes



June 2, 1970 s. 'LANGE CHARGI NG PROCESS FOR ELECTROSTATIC SCREENING OF COLOR TUBES Filed March 16. 1966 I N VEN TOR. k leword G. Lon

At'ror United States Patent 3,515,548 CHARGING PROCESS FOR ELECTROSTATIC SCREENING OF COLOR TUBES Howard George Lange, Chicago, Ill., assignor to Zenith Radio Corporation, a corporation of Delaware Filed Mar. 16, 1966, Ser. No. 534,893

Int. Cl. G03g 13/22 US. Cl. 961 3 Claims ABSTRACT OF THE DISCLOSURE CROSS-REFERENCE TO RELATED TO APPLICATION The screening process of this application is a further development of the electrostatic process described and claimed in applicants copending application, Ser. No. 481,316, filed Aug. 20, 1965 and assigned to the assignee of the present invention.

The present invention is concerned with a process of electrophotographically or electrostatically screening the image area of a color cathode-ray tube and is addressed most particularly to the process by which a corona or similar device is energized in establishing the necessary charge condition preparatory to imaging.

The se of electrostatic screening in producing the image area of a color cathode-ray tube is described, to applicants knowledge, for the first time in his copending application, Ser. No. 481,316, filed Aug. 20, 1965 and assigned to the assignee of the present invention. The present application is a continuation-in-part of the earlier filed application.

As directed in the earlier disclosure, electrostatic screening of a color cathode-ray tube is accomplished by applying to the image area a conductive layer and a superposed photoconductive layer. Thereafter, a coorna device scans the photoconductive layer in order to establish thereon a uniform charge and a charge image is then established by exposing selected portions of the photoconductor to actinic light. Where the tube is of the similar dot triad type, the areas of the photoconductor subjected to actinic light are determined by exposing the layer through the aperture mask in essentially the same way that the image area is exposed in the more customary photoresist screening process. The areas of the photoconductor upon which actinic light impinges become discharged and the application of a toner composition, including a phosphor of one of the primary colors, permits depositing or screening that phosphor on the discharged areas of the photoconductor. Following this procedure three times, with suitable adjustment of the position of the exposing light source for each of the cycles, produces the dot triads of the color tube.

The present invention has to do with energizing the corona device in an improved manner so that the photoconductive layer receives a uniform charge prior to exposure.

Accordingly, it is an object of the invention to provide an improved energization of the corona device in the process of electrostatically screening a color cathode-ray tube.

It is a specific object of the invention to control energization of the corona device to achieve uniformity of the charge on the photoconductive layer and also to minimize cross contamination of the several phosphor colors with respect to one another.

More specifically, the invention is applicable to the process of electrostatically screening the image area of a color cathode-ray tube to establish thereon interlaced patterns of phosphors of different colors, in which process a layer of photoconductive material is applied over the image area and is charged by a corona device preparatory to screening phosphor on selected portions of the layer. The invention concerns the improvement which comprises energizing the corona device to a predetermined potential level in screening the image area 'with one of the phosphors and thereafter energizing that device to a different potential level in screening the image area with the next or another of the phosphors. More specifically, the energizing potential is successively reduced in the screening of the several different phosphors.

In accordance with another aspect of the invention, the corona device is energized with both an alternating current and a direct current component of potential. Furthermore, the invention features reducing the direct current component of the energizing potential successively in the screening of the several phosphors.

The features of the present invention which are believed to be novel are set forth with particularly in the appended claims. The invention, together with further objects and advantages thereof, may best be understood by reference to the following description taken in connection with the accompanying drawing, the single figure of which represents schematically a portion of the face panel of a color cathode-ray tube and a corona device with a power supply to effect energization in accordance with the invention.

A color cathode-ray tube lends itself to electrostatic or electrophotographic screening since the envelope is a two piece structure having a screen or cap section and a companion funnel or cone section. The cap may be likened to a flanged dish and it may have any desirable cross section but is usually round or rectangular. The cone has the same cross section as the cap and is dimensioned so that its large end may fit precisely with the free end of the flange of the cap section to facilitate their integration by frit sealing. The opposite or small end of the cone accommodates the neck of the tube which houses the electron gun or guns for generating scanning electron beams. Since this application is addressed to screening, the remaining description will be confined to processing the cap section in order to establish a tri-color screen of the dot triad type.

The cap section and in particular the innersurface thereof which constitutes the image area of the tube is first made chemically clean in any well-known manner and is then provided with a conductive layer which has a resistivity less than 10 ohms per square unit and preferably less than 10 ohms per square unit; the lower the resistivity the better. A suitable material for the conductive layer comprises a polymer of a predominant amount of a vinylbenzyl quaternary ammonium compound as described in US. Pat. 3,011,918, issued Dec. 5, 1961 to Lloyd H. Silvernail et al. Such a material is available commercially from the Dow Chemical Company under the trade designation QX-261l.7. A quantity of this material diluted with water is spin coated over the panel of the tube in process at a speed of about 60 rpm. to apply a uniform coating of approximately 0.003 inch thickness over the entire screen surface. This layer is dried with heat.

The next step in the screening process is the applica- 3 tion over the image area of a layer of a photoconductive material superposed on the conductive layer. A formulation for this layer which has been successfully employed is as follows:

M1. solution of polyvinyl carbazole in monochlorobenzene 150 0.154% solution of anthroquinone in monochlorobenzene 30 10% solution of a plasticizer in monochlorobenzene- 20 Monochlorobenzene 460 A commercial form of polyvinyl carbazole is obtainable under the designation M-170 from Badische Anilin & Soda Fabrik A.G. A suitable commercially available plasticizer is Plastolein 9066LT marketed by Emery Industries, Inc.

The photoconductive layer may be applied by spinning, flowing, spraying or the like and the desired viscosity may be obtained by the amount of monochlorobenzene employed. This layer may have a thickness of approximately 0.0003 inch and is preferably air dried.

Having formed the photoconductive layer, the preferred next process step is pretreating that layer, as taught in a copending application of Joseph C. Drozd, Ser. No. 534,633, filed Mar. 16, 1966, to establish a substantially uniform reference charge condition therein. This may be accomplished by pre-exposing and/or prerinsing the layer with a surfactant, preferably the same surfactant that is used in the toner employed in developing the charge image to be considered presently. A formulation for a prerinse that has been successfully employed is as follows:

Ml. Trifluorotrichloroethane (Freon TF) 100 10% solution of a surfactant in Freon TF .5

A suitable surfactant which is available commercially is designated L101 Pluronic marketed by Wyandotte Chemical Company. After the rinse, the photoconductive layer is air dried and the pretreatment of the photoconductive surface is now completed.

The next step in the screening process involves establishing a uniform charge on the surface of the photoconductive layer and this process step of the invention will be explained with reference to the figure wherein 10 designates a segment of a glass substrate, specifically, the panel of the color tube that is being screened. The conductive layer disposed on the image area of the panel is designated 11 and 12 is the superposed photoconductive layer. Charging is accomplished by a corona device which of itself may be of conventional construction but in accordance with the invention is energized in a unique fashion. As illustrated in the drawing, the charging device has a grid of conductors 13 supported within a grounded shield or metallic shroud 14. The conductors may be segments of two-mil tungsten wires with /2 inch spacing between the conductors and approximately the same spacing from the conductors to the surface of the photoconductive layer. The charging device is slidably supported from a frame member 15 to facilitate scanning the tube panel as required to charge the entire surface of photoconductor 12.

The corona device is energized with both direct current and alternating current components of potential delivered from an A.C. source 17 and a DC. source 18. In practicing the invention, the energizing potential of the corona device is modified as the screening of the several phosphors takes place; in particular, the corona device is energized to successive lower potentials in the screening of the different phosphors. This is indicated in the drawing by the provision of three output terminals 19, 20 and 21 of DC. source 18 and a switch which has a movable blade 22 for selectively engaging one of these output terminals. The switch is coupled through a transformer 23 to a lead 24 which is insulated from shield 14 but is in circuit connection with the grid conductors 13. One winding of transformer 23 connects with the output of A.C. source 17.

In carrying out the charging step of the screening process, the corona device is energized to a maximum potential from sources 17, 18 in establishing the charge preparatory to screening with the first phosphor which, for convenience, will be considered as the green phosphor. Where A.C. and DO. energization is employed, as indicated in the drawing, it is essential that the amplitudes be sufficient so that collectively they may effect ionization of the air within the shroud. It is convenient to arrange that the A.C. component of energizing potential have a peak amplitude to at least effect incipient ionization and indeed the amplitude may be sufiicient to accomplish ionization. The DC. component of energization serves to create a field which accelerates or directs the resulting ions to the surface of the photoconductive layer. Good results have been obtained by charging the photoconductive layer for three minutes with an A.C. component of 4.3 kilovolts RMS and a DC. component of two kilovolts obtained at terminal 19. During charging, the corona unit continuously scans over the image area to effect uniform charging.

After the photoconductive layer has been charged by the corona device, and assuming that the tube is of the dot triad type, a charge image is established on the photoconductive layer by exposing that layer to actinic light through the shadow mask of the tube. This exposure takes place in a lighthouse with a source of actinic energy or light positioned to simulate the center of deflection of the electron gun to energize the phosphor elements of the particular color instantaneously being applied, green for the case under consideration. A correcting lens may be interposed in the optical system to compensate for possible errors of registration of the beam relative to the phosphor elements as disclosed in Pat. 3,003,874, issued Oct. 10, 1961 in the name of Sam H. Kaplan. This exposure develops on photoconductor 12 a latent negative charge image of the dot areas of the screen that are to be assigned to green.

The next step is the development of that image with a toner comprising a carrier liquid, green phosphor and a surfactant. The surfactant establishes a charge on the phosphor particles of the same polarity as the charge of the photoconductive layer so that the phosphor ingredient of the developer is rejected from the charged portions of the layer and is deposited in the uncharged areas. In this fashion, the deposit of green phosphor dots takes place and may be afiixed by heat or a fixing agent as described in the Lange application.

Screening with the next color phosphor now takes place and this may be initiated by a pretreatment of a rinse or pre-exposure as described in the above-identified Drozd application. In charging photoconductive layer 12, preparatory to establishing the latent image of a second phosphor, switch 22 is displaced to contact terminal 20 of source :18. The photoconductor is now charged with the same component of A.C. but with a reduced component of DC. and for the same charging interval. In practicing the invention, the potential available at terminal 20 is adjusted to one kv. for screening of the second phosphor. The procedural steps following charging of the photoconductive layer include exposing through the mask, developing and fixing as described in the Lange application and also as recited above.

Finally, in depositing the third color phosphor switch 22 is positioned to terminal 21 so that in the charging process energization of the corona device is with the same fixed amount of A.C. while the DC. component is now approximately 500 volts. Again, the charging time is about three minutes and once layer 12 has been charged, it is exposed and developed to screen the last of the three phosphor colors.

In previous methods of electrophotography, it has been customary to charge the photoconductive layer by means of a corona device energized by a high direct current potential. Experience shows that with certain organic photoconductors charged in this manner there may be spurious images produced in the developing steps. While the reason for such spurious images is not fully understood, it is believed that possibly the current density produced by the corona device is so high as to have a .destructive effect on the photoconductor resulting in a nonuniform distribution of charge. There may, for example, 'be localized heating of areas of the photoconductor and these disadvantages are substantially minimized in the described process which contemplates reducing the energization of the corona device in screening with the several phosphors. It is also proposed that there be an alternating current component of energizing potential to achieve the ionization for charging which permits reducing the D.C. component only to the value necessary for directing the ions to the photoconductive layer. It is expected that the phosphor dots deposited on the photoconductive layer represent localized areas with a dielectric constant, volume resistivity and other parameters differing from that of the photoconductor. Charging in the face of these variations is believed to produce field gradients which may result in the phosphors being directed to undesired areas and manifesting highly undesirable contamination of one phosphor into the next. The described process of reducing the charging voltage minimizes such gradients and also cross contamination. While the overall charge acceptance of the photoconductive layer is, likewise, reduced the desired field gradients produced by photoconductive discharge in the exposure step are sufiicient to achieve the required degree of dot phosphor density.

As indicated by the legend on source 17, the AC. component may be an alternating current potential or a pulse of potential. Acceptable results have been obtained using 60 cycle alternating current or, alternatively, using pulses of a 60 p.p.s. repetition rate and having a pulse amplitude approximately equal to the RMS amplitude of the AC. potential.

Reference may be had to the aforesaid Lange application, Ser. No. 481,316 for illustrative formulations of conductive, photoconductive layers, and toner or developers employed in electrophotographic screening. These formulations, per se, constitute no part of the present invention which is addressed to energization of the corona device.

While particular embodiments of the invention have been shown and described, it will be obvious to those skilled in the art that changes and modifications may be made without departing from the invention in its broader aspects, and, therefore, the aim in the appended claims is to cover all such changes and modifications as fall within the true spirit and scope of the invention.

I claim: 1

1. In the process of electrostatically screening the image area of a color cathode-ray tube which comprises applying over said area a layer of a volatilizable photoconductive material, and screening thereon interlaced patterns of phosphors of different colors by the following process steps for each such phosphor: (a) charging said layer with a corona device to establish a substantially uniform level of charge on said photoconductive layer, (b) exposing said photoconductive layer with light through transparent portions of a color selection electrode to establish a latent charge image thereon and (c) applying to said photoconductive layer a quantity of phosphor particles of a given color to develop said latent image, the improvement which comprises:

applying to said corona device in screening said image area with one of said phosphors a first predetermined potential having a first component to elfect ionization of the atmosphere over said image area and having a DC. component to establish an electric field for directing charged ions to said image area;

and applying to said corona device in thereafter screening said image area with another of said phosphors a similar potential likewise having first component and a D.C. component but with its D.C. component substantially reduced in amplitude compared to said D.C. component of said first potential.

2. The process of electrostatic screening in accordance with claim 1 in which said first component is an alternating current potential.

3. The process of electrostatic screening in accordance with claim 2 in which the amplitude of said direct current potential is approximately half that of said alternating current potential.

References Cited UNITED STATES PATENTS 3,212,887 10/1965 Miller et al 96-1 3,329,590 7/1967 Renfrew 20418 3,337,340 8/1967 Makan 96-1 FOREIGN PATENTS 610,657 12/ 1960 Canada.

GEORGE F. LESMES, Primary Examiner I. C. COOPER HI, Assistant Examiner US. Cl. X.R. 

